1. Field of the Invention
The present invention relates to the field of current sense amplifiers used for measuring current.
2. Background of the Related Art
Current sense amplifiers are typically used to measure the amount of current supplied by and to a power supply in various types of electronic equipment. Several conventional approaches to the implementation of current sense amplifiers exist. These include a single polarity low side current sense amplifier, a low side current sense amplifier with bipolar sensing and a high side switching current sense amplifier.
FIG. 1 depicts a schematic diagram of a conventional single polarity low side current sense amplifier. In this conventional approach, the current sense amplifier 10 comprises a sense resistor (R.sub.SENSE), two gain resistors (R.sub.1 and R.sub.2 ), and an operational amplifier (Op Amp). The operational amplifier has an input common mode operating range that includes its negative input terminal. The output signal provided by the operational amplifier is a voltage that may be expressed as follows: EQU V.sub.OUT =ILOAD*R.sub.SENSE *R.sub.2 /R.sub.1.
The current sense amplifier 10, however, can only measure current flowing in one direction, since current flowing in the opposite direction pulls one input of the operational amplifier below the negative input terminal. To obtain a bipolar output signal, an offset reference voltage Vref may be added to the operational amplifier, as illustrated in FIG. 2. However, this design adds a reference-dependent offset to the amplifier output.
A different approach is required for high side switching. If the operational amplifier in FIG. 1 is inverted, its output signal will be referenced to the high or the positive side of the battery. This is rarely desired, since conventionally V.sub.OUT is referenced to ground. FIG. 3 shows a high side circuit that can measure the voltage drop across R.sub.SENSE for current flow in either direction. If R2 was grounded, current flow could only be measured in one direction, as flow in the reverse direction would require an amplifier output below the negative input terminal. With R2 connected to V.sub.REF, the output voltage becomes EQU V.sub.OUT =V.sub.REF +ILOAD*R.sub.SENSE *R.sub.2 /R.sub.1 where R.sub.4 /R.sub.3 =R.sub.2 /R.sub.1.
A disadvantage of this circuit is that it requires close matching of the foregoing resistance ratios of resistors R.sub.1 through R.sub.4 in order to suppress common mode signals.
Accordingly, there is a need in the technology to provide a current sense amplifier that can measure the magnitude and polarity of current flow, without the need to accommodate an offset output signal nor the need to monitor a plurality of gain resistors.